Ion-ionophore interactions in polymeric membranes studied by thin layer voltammetry

In ion sensing applications, selective ion-receptor complexation is the molecular basis for endowing the sensing material with selectivity. In this work, thin polymeric membrane-based ion transfer voltammetry is used to investigate ion-receptor complexation, using a range of electrically neutral ionophores and surfactants as examples. Previous studies lacked a convincing approach to eliminate the influence from transducing layer, resulting in deviations of the observed binding constants compared to potentiometric methods. A recently developed method allows for subtracting the potential changes of the transducing layer, thereby overcoming this challenge. Using this approach, a range of ionophores are assessed. Valinomycin for the detection of potassium gave a logarithmic complex formation constant in the membrane of 9.69 +/- 0.25 with a 1:1 stoichiometry. Lithium ionophore VI for lithium gave a logarithmic stability constant of 5.97 +/- 0.06 with 1:2 complexes; while sodium ionophore X for sodium (7.57 +/- 0.03, 1:1) and calcium ionophore IV for calcium (21.57 +/- 0.25, 1:3) were also characterized, in addition to their complexes with potential interfering ions. The complex formation of three surfactants with potassium are also explored in membranes containing valinomycin, with Brij-35 (4.88 +/- 0.08, 1:1), Triton X-100 (5.63 +/- 0.10, 1:1), F-127 (4.63 +/- 0.49, 1:1). Limitations of the approach are discussed, which includes the need for electrochemical reversibility and a sufficiently high lipophilicity to adequately retain the components in the membrane

Automated summary

This study investigates the ion-receptor complexation using thin polymeric membrane-based ion transfer voltammetry. A convincing approach is introduced to eliminate the influence from the transducing layer, leading to accurate binding constants. Various ionophores and surfactants are assessed, along with their complexes with potential interfering ions. The limitations of the method, such as the need for electrochemical reversibility and sufficient lipophilicity, are discussed.